Radiocommunication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched bearers in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architectures will likely evolve toward all Internet Protocol (IP) networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructures and would, therefore, typically prefer to migrate gradually to all IP network architectures in order to allow them to extract sufficient value from their investment in existing infrastructures. Also to provide the capabilities needed to support next generation radiocommunication applications, while at the same time using legacy infrastructure, network operators could deploy hybrid networks wherein a next generation radiocommunication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network. Alternatively, a radiocommunication system can evolve from one generation to the next while still providing backward compatibility for legacy equipment.
One example of such an evolved network is based upon the Universal Mobile Telephone System (UMTS) which is an existing third generation (3G) radiocommunication system that is evolving into High Speed Packet Access (HSPA) technology. Yet another alternative is the introduction of a new air interface technology within the UMTS framework, e.g., the so-called Long Term Evolution (LTE) technology. Target performance goals for LTE systems include, for example, support for 200 active calls per 5 MHz cell and sub 5 ms latency for small IP packets. Each new generation, or partial generation, of mobile communication systems add complexity and abilities to mobile communication systems and this can be expected to continue with either enhancements to proposed systems or completely new systems in the future.
LTE uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station—typically referred to as an eNB in LTE—transmits control information indicating to which terminals and on which resource blocks the data is transmitted during the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as the control region is illustrated in FIG. 3.
Data is transmitted to the terminal in the form of either one or two transport blocks per subframe. Each transport block is segmented into code block according to section 5.1.2 in 3GPP Technical Specification 36.212 V10.0.0. Each code block is coded and rate matched according to 3GPP Technical Specification 36.212 V10.0.0. The rate matched and coded bits are then concatenated per associated transport block according to 3GPP Technical Specification 36.212 V 10.0.0, which is referred to as code block concatenation. This sequence of coded and rate matched bits that corresponds to one transport block after code block concatenation is referred to as one codeword. LTE uses hybrid-ARQ (HARM) where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NACK). In the case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data to the terminal or user equipment (UE). In LTE systems, HARQ with incremental redundancy is used. Thus, instead of re-transmitting the same portion of the codeword, different redundancy versions are re-transmitted yielding an extra gain over, for example, Chase combinin.
Ideally, a buffer of sufficient size should be available at the receiver side for HARQ transmissions and retransmissions so that the received soft values for entire codewords can be stored. However, due to terminal complexity and cost concerns, the soft buffer size in a terminal is typically limited. For example, for higher rate transmissions, i.e., where larger codewords are sent from the transmitter, the size of a UE's buffer may not be sufficient to store the complete codeword. Therefore, the eNB and UE should both have knowledge about the soft buffer size since otherwise the eNB may transmit coded bits the UE cannot store.
FIG. 4 depicts simplified a complete codeword 400, including both systematic bits and parity bits, and also shows how many soft bits the terminal can store for a given soft buffer size relative to the codeword size, according to one illustrative example. If the eNB and terminal have the same understanding about the soft buffer size, then the eNB will not transmit coded bits the terminal cannot store. Instead, the eNB only takes those coded bits that are stored by the terminal and uses those bits for transmissions or re-transmissions.
This aspect of HARQ-related buffering of codewords can be depicted by a circular buffer 500, an example of which is shown in FIG. 5. Like FIG. 4, each buffered transmission or re-transmission will typically include both systematic bits 502 and parity bits 504. Note that in this example, the complete circle 500 corresponds to the soft buffer size and not to the entire codeword. In a first transmission, depending on the code rate, some/all of the systematic bits 502 and none/some parity bits 504 are transmitted. In a retransmission, the starting position is changed and bits corresponding to another part of the circumference of the circular buffer 500 are transmitted.
In Rel-8 LTE FDD, each terminal has up to 8 HARQ processes per component carrier and each HARQ process can contain up to two sub-processes for supporting dual-codeword MIMO transmissions. The design set forth in Rel-8 LTE is to divide the available soft buffer equally into the configured number of HARQ processes. Each portion of the divided soft buffer can then be used to store soft values of the received codewords. In the case of dual-codeword MIMO transmission, the divided soft buffer shall be further divided equally to store the soft values of the two received codewords. These aspects of soft buffer allocation are described in more detail in the standard document 3GPP Technical Specification 36.212, Section 5.1.4.1.2 “Bit collection, selection and transmission”, the disclosure of which is incorporated here by reference.
The soft buffer allocation described in the above-incorporated by reference standard document for the single-codeword transmission modes in Rel-8 of LTE is conceptually illustrated in FIG. 6. Note that in FIG. 6, there is a buffer SB0-SB7 reserved for each codeword. The soft buffer allocation for the dual-codeword transmission modes described above for Rel-8 of LTE is illustrated in FIG. 7. Note that in FIG. 7, the buffer reserved for each codeword is only half the size of the previous operating case of FIG. 6. This illustrates that the soft buffer limitation problem is particular acute in dual-codeword MIMO transmission operations. This limitation reduces the effectiveness of soft combining gains from incremental redundancy retransmissions.
The LTE Rel-8 standard has recently been standardized, supporting bandwidths up to 20 MHz. In order to meet the upcoming IMT-Advanced requirements, 3GPP is currently standardizing LTE Rel-10, also referred to as “LTE-Advanced”. One property of Rel-10 is the support of bandwidths larger than 20 MHz while still providing backwards compatibility with Rel-8. This is achieved by aggregating multiple component carriers, each of which can be Rel-8 compatible, to form a larger overall bandwidth to a Rel-10 terminal. For example, as shown in FIG. 8, five component carriers (CCs) 800 can be aggregated into a single 100 MHz bandwidth. Note that carrier aggregation can involve contiguous intra-band carrier aggregation, non-contiguous intra-band carrier aggregation, and/or inter-band carrier aggregation.
In LTE-10 each component carrier will operate with its own set of HARQ processes. Since the total soft buffer memory needs to be shared among the component carriers, the soft buffer size per component carrier can vary depending on the number of configured component carriers and the number of configured MIMO transmission modes for each component carrier. The available soft buffer size for each codeword also depends on how the soft buffer is divided and allocated amongst all codewords.
Thus, the issues described above with respect to soft buffer sizes and allocations are expected to be exacerbated in Rel-10 of LTE with the addition of carrier aggregation.
ABBREVIATIONS/ACRONYMSRel-8Release-8Rel-10Release-10ACKAcknowledgementARQAutomatic Repeat RequestCACarrier AggregationCCComponent CarrierFDDFrequency Division DuplexingHARQHybrid Automatic Repeat RequestIMTInternational Mobile TelecommunicationsLTELong term evolutionMIMOMultiple-Input Muliple-OutputNAKNon AcknowledgementNACKNon AcknowledgementOFDMOrthogonal Frequency Division Multiple AccessUEUser equipment